Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Examples of wireless communication systems are Long Term Evolution (LTE), Universal Mobile Telecommunications System (UMTS) and Global System for Mobile communications (GSM).
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
One important function of 3GPP cellular radio technologies is the control of user mobility by using the Radio Resource Control (RRC) and X2AP protocols. The network controls the handover of UEs in RRC Connected mode from one cell to another whereas a UE in idle mode performs cell selection and reselection itself.
3GPP has recently agreed within the scope of low-cost Machine Type Communication (MTC) work item to introduce a new UE category termed as category 0. The UE category defines a combined set of UE capabilities. A draft Change Request (CR) for the introduction of category 0 is available in R2-140964 as presented during RAN2#85 meeting in Prague, 10.02.2014 to 14.02.2014, Prague, Czech Republic.
The UEs of this new category are less capable than e.g. UEs of legacy category 1, which currently is the least capable legacy category. Legacy category UEs are UEs that implements an older version of the RRC protocol than the new category UE, such as category 0 UE, does. Upon handover of a category 0 UE to a legacy target eNB, i.e an eNB that implements an older version of the RRC protocol than the new category UE, e.g. category 0 UE, does, the handover preparation will be successful, while the subsequent handover will fail because the legacy eNB and the new category UE, e.g. the category 0 UE, are only partly compatible when it comes to the RRC protocol. Following the handover failure the category 0 UE will experience a radio link failure. Subsequently, the category 0 UE will try to trigger RRC Connection Re-establishment which also will fail for the same reason of non-compatibility. Finally, the category 0 UE will transition to idle mode where it will search for suitable cells. The failed handover and the unsuccessful RRC Connection Re-establishment will result in unnecessary signalling in the network and in shorter battery lifetime for the UE. Connection drops may also result.